Nitrogeneous components, in particular, ammonium nitrogen in nitrogeneous liquors constitute a causal origin for eutrophication of rivers, lakes and seas and, thus, have to be removed efficiently during wastewater treating processes. In general, organic nitrogeneous components in liquors are subjected to bacterial biolysis and are converted mostly into ammonium nitrogen. Conventional processes for treating nitrogeneous liquors are mainly based on microbiological nitrification-denitrification techniques. Such processes for microbiologically treating nitrogeneous liquor consist usually of two biological reaction steps, one is a nitrification step, in which the ammonium nitrogen is oxidized by ammonia-oxidizing bacteria into nitrite nitrogen and this nitrite nitrogen is further oxidized by nitrite-oxidizing bacteria into nitrate nitrogen, and the other is a denitrification step, in which the nitrite and/or nitrate nitrogen are converted into nitrogen gas by microbiological reaction by denitrifying bacteria, which belong to heterotrophic bacteria, under a metabolism utilizing an organic substance, such as methanol or the like, as an electron donor. Such nitrification-denitrification processes require specific organic substance, such as methanol, in addition to a large consumption of electric power for maintaining sufficient aeration for oxidizing the ammonium nitrogen to nitrite nitrogen and, further, to nitrate nitrogen.
As an improved treating technique for remedying the above-mentioned shortcomings, there has been proposed by, for example, Patent Document 1, a so-called denitrification treatment method using anammox bacteria, which belong to autotrophic bacteria. This method consists in a biological denitrification using a group of autotrophic denitrifying microbes capable of reacting ammonium nitrogen as the electron donor with nitrite nitrogen as electron acceptor under anaerobic condition to form nitrogen gas. In this process, addition of organic substance can be dispensed with and consumption of electric power for aeration for nitrification could be kept at very lower level because the oxidation is restricted to the range wherein a part of ammonia is oxidized only into nitrite. Moreover, autotrophic microbes grow at a lower growth yield, so the excess sludge is considerably small as compared with that from heterotrophic microbes. However, the above technique has a shortcoming of requiring a large size apparatus, since the ammonia oxidation (nitrite formation) and the autotrophic denitrification are realized in each isolated vessel.
In Patent Document 2, there is disclosed a technique for effecting denitrification of nitrogeneous liquor by using a specific combination of anammox bacteria, belonging to autotrophic bacteria, and ammonia-oxidizing bacteria, both supported on a granulated porous carrier having communicating pores, such as a sponge, in which a part of the ammonium nitrogen in the liquor is oxidized into nitrite nitrogen by the ammonia-oxidizing bacteria by performing the biological reaction while supplying an oxygen-containing gas at a rate not obstructing the growth of the anammox bacteria and, at the same time, denitrification is realized by the anammox bacteria. This technique consists in simultaneous parallel bioreactions for the ammonia oxidation and for the denitrification performed by supplying continuously the nitrogeneous liquor to be treated and the oxygen-containing gas to the reaction vessel. Here, the bioreactions can only proceed when the liquor contains ammonium nitrogen and, therefore, the effluent always contains ammonium nitrogen, which means that the degree of removal of ammonium nitrogen is not high. Furthermore, it suffers from a shortcoming that control of the oxygen supply rate using an oxygen-containing gas is so difficult that the anammox reaction may not proceed at a sufficiently maintained rate due to over-growth of nitrite-oxidizing bacteria functional to oxidize the nitrite nitrogen into nitrate nitrogen causing a deficiency of nitrite, when the oxygen supply rate is caused occasionally to be high.
Patent Document 1: WO89/07089 (Japanese Patent Kohyo Hei 3-501099 A)
Patent Document 2: Japanese Patent Kokai 2001-293494 A